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bad idle when hot

4111 Views 10 Replies 4 Participants Last post by  Mark / Ohio
Picked up a walk behind lawnmower from my mom, said she was having problems with it idling funny. Maintenance had never been done to it, so i replaced the spark plug, air filter and gave it some fresh oil and gas hoping that it would solve the problem, and of course it didn't. I have not replaced the blade, even though it is beat to **** and needs to be replaced.

So here is the deal. When i first start and run the mower, it runs great, good idle and everything. But after running it for a while the idle starts to go up and down.

Anyone know what could be causing this issue and how to fix it?
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:welcome: To The Forum!! What is the model name and model # of the Toro mower you are working on? What type of engine is on the mower?
Ya i guess that would be important information huh haha. Sorry for the delay, finals week :(

It's a 6.0HP GTS 21" Personal Pace Recycler, model # 20022. Bought back in 1999.
engine is a Briggs and Stratton Model # 12H802-1767-E1
Try loosening the fuel cap when it is acting up and see if it smooths back out. Vent might be partly plugged.
how would i go about clearing the vent if it is clogged?
how would i go about clearing the vent if it is clogged?
Find something small and sharp that you can use to stick through the hole such as sewing needle/toothpick etc.

Depending on the application, you may need to also remove the gasket on the bottom of the cap for better access vs. doing it through the top.
Ok, should i just go through the whole exhaust system and make sure its no clogged up?
Being a 4-cycle engine it is pretty unlikely that there is an exhaust blockage. My bet is a lean fuel mixture. Could be carburetor but since it runs ok until warm was my reasoning for thinking maybe fuel tank drawing a vacuum from a plugged vent.
What other possible vents may there be. I took a look at the gas cap and i did not see an sort of vent hole.
I took a quick look at the cap on my Toro/Briggs which is of the same vintage and did not see a hole in it either. The only thing I noted was two tiny "V" groves 180° apart from each other in the gasket in the cap that they might be using as the vent which then pulls enough air around the threads to fill the empty space as the fuel level drops.

It's possible they might have rubber valve built into the tank some place like my old Homelite chain saw but I kind of doubt it. (I learned that one day while cutting a branch in a tree and feeling a cool liquid dripping on my leg after the vent had rotted and fell out of the tank. :eek: )
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